It is well known that exposure to sunlight can pose a number of hazards to the skin. These damaging effects may result not only from sunbathing but also from the sunlight exposure associated with daily outdoor activities. The major short term hazard of prolonged exposure to sunlight is erythema, i.e., sunburn, which primarily results from UVB radiation having a wavelength of from about 290 nm to about 320 nm. Over the long term, however, malignant changes in the skin surface often occur. Numerous epideminologic studies demonstrate a strong relationship between sunlight exposure and human skin cancer. Another long term hazard of ultraviolet radiation is premature aging of the skin, which is primarily caused by UVA radiation having a wavelength of from about 320 nm to about 400 nm. This condition is characterized by wrinkling and pigment changes of the skin, along with other physical changes such as cracking, telangiectasis, solar dermatoses, ecchymoses, and loss of elasticity. The adverse effects associated with exposure to UV radiation are more fully discussed in DeSimone, "Sunscreen and Suntan Products," Handbook of Nonprescription Drugs, 7th Ed., Chapter 26, pp. 499-511 (American Pharmaceutical Association, Washington, D.C.; 1982); Grove and Forbes, "A Method for Evaluating the Photoprotection Action of Sunscreen Agents Against UV-A Radiation," International Journal of Cosmetic Science, 4, pp. 15-24 (1982).
As a result of the abovementioned hazards associated with sunlight exposure, the general public's interest in the sun protection product market has grown considerably. Today, there are not only sunscreen products for sunbathing but there are also a variety of personal care products containing sunscreens, particularly cosmetic type products which are worn daily. "Personal care products" refer to health and cosmetic beauty aid products generally recognized as being formulated for beautifying and grooming the skin and hair. For example, personal care products include sunscreen products (e.g., lotions, skin creams, etc.), cosmetics, toiletries, and over-the-counter pharmaceutical products intended for topical usage.
Many conventional sunscreen products, in particular, are deficient, however, due to their inability to provide efficacious protection against broad spectrum UV radiation, i.e., protection against both UVB and UVA radiation. Today, most commercially available sunscreen products are efficient at absorbing UV radiation in the 290 nm to 320 nm UVB region such that sunburn of the skin is prevented. They are less efficient when it comes to absorbing light which falls in the 320 nm to 400 nm UVA region, which leaves the skin vulnerable to premature skin aging. This deficiency is due in part to the limited number of UVA absorbing sunscreen actives which are both commercially available and approved for global use. One class of these sunscreen actives includes dibenzoylmethane compounds which provide broad spectrum UV protection and are described in U.S. Pat. No. 4,387,089, issued to De Polo on Jun. 7, 1983 and U.S. Pat. No. 4,489,057, issued to Welters et al. on Dec. 18, 1984. 4-tert-butyl-4'-methoxydibenzoylmethane, in particular, is approved for global use. Unfortunately, when used individually in sunscreen products, most dibenzoylmethane compounds tend to photodegrade upon exposure to UV radiation thereby reducing their UVA efficacy. Furthermore, it is well known that even when dibenzoylmethane compounds are used in combination with other sunscreen actives, severe photodegradation still tends to occur. For instance, U.S. Pat. No. 4,387,089 proposes to combine 4-(1,1 dimethylethyl)-4'-methoxydibenzoylmethane with various UVB sunscreen actives, namely 2-ethylhexyl-p-methoxycinnamate (also referred to as octyl methoxycinnamate), for the purpose of absorbing all of the UV radiation between 280 and 380 nm and in which case it is stated that the ratio of the dibenzoylmethane sunscreen active to the UVB sunscreen active is not critical. In fact, it is well known that the combination of about 1-3% 4-tert-butyl-4'-methoxydibenzoylmethane and 6-7.5% 2-ethylhexyl-p-methoxycinnamate results in a formulation in which both of the sunscreen actives are photounstable which in turn makes the overall formulation photounstable. Without being limited by theory, it is believed that upon exposure to UV radiation, 4-tert-butyl-4'-methoxydibenzoylmethane and 2-ethylhexyl-p-methoxycinnamate react with each other to form non-UV absorbing compounds.
Therefore, sunscreen products which include dibenzoylmethane sunscreen actives are typically more difficult to formulate due to their inherent lack of photostability. One approach to stabilize these types of sunscreens is described in U.S. Pat. No. 5,783,174 issued to Deckner on Jul. 21, 1998, which involves the use of benzylidene camphor sunscreens to stabilize the dibenzoylmethane compound. Such compositions, however, are not currently approved for global use on humans.
Thus, there is a need for photostable compositions suitable for providing protection against the harmful effects of UV radiation to human skin. In particular, in the personal care industry, a need remains for sunscreen products having excellent photostability, efficiency, and which provide broad spectrum UV protection (i.e., against both UVA and UVB radiation) in a safe and economical manner.
Likewise, in other industries such as the automotive care, marine vehicle care, household care, animal care, and coatings industries, UV radiation causes undesirable wear and damage. Therefore, a need exists for photostable compositions suitable for providing protection against the harmful effects of UV radiation to a variety of objects and/or materials prone to sun exposure.
In contrast to the common knowledge of those having ordinary skill in the art, Applicant has surprisingly found that the compositions of the present invention, which comprise a UVA-absorbing dibenzoylmethane sunscreen active, a photostabilizing system which includes 2-ethylhexyl-p-methoxycinnamate, and a suitable carrier such that the mole ratio of 2-ethylhexyl-p-methoxycinnamate to UVA-absorbing dibenzoylmethane sunscreen active is from about 0.15:1 to about 1:1, provide excellent photostability, efficiency, and UV protection efficacy (including both UVA and UVB protection), in a safe and economical manner.